The present invention relates to a frequency spectrum analyzer for evaluating the frequency spectrum of an input signal with high accuracy by subjecting digital data corresponding to the input signal to Discrete Fourier Transform (DFT).
A conventional frequency spectrum analyzer is such that a series of digital data corresponding to an input signal (if the input signal is an analog signal, the digital data can be obtained after the input signal has been subjected to A/D conversion) is multiplied by a window function and then subjected to DFT; an absolute value of the converted complex number data is calculated; and the calculated value is displayed as a frequency spectrum at discrete frequency points.
According to the conventional frequency spectrum analyzer, when a user specifies desired frequency points, the frequency spectrum at the frequency points is displayed as a Root-Mean-Square value (hereinafter referred to as "RMS") of the amplitudes, or decibel values at the frequency points are displayed together with the frequency on the basis of a predetermined value at a reference frequency.
However, e.g., in the case where steady-state vibration of a rotating body is subjected to frequency analysis to evaluate the spectrum of a predetermined frequency using the conventional frequency spectrum analyzer, the numbers of rotations of the rotating body tend to be shifted due to some cause every time it is measured, so that the amplitudes of the spectrum are varied, thereby impairing the reliability of the measured values.
Specifically, e.g., in the case where a rectangular window is used as a window function, if a frequency f of an input signal coincides with a discrete frequency point k; i.e., if f.sub.0 is the frequency corresponding to a frequency point k=k.sub.0 as shown in FIG. 5(a), a single spectral line is observed when f=f.sub.0. However, if the frequency f of the input signal does not coincide with the frequency point; e.g., the frequency f is varied to the midpoint (f.sub.0 +.DELTA.f/2) between the frequency points, the spectrum of the frequency is dispersed into a plurality of spectral lines (hereinafter referred to as "leakage") as shown in FIG. 5(b), so that the amplitude of the spectrum at the target frequency point k.sub.0 is undesirably reduced to 64% (-3.9 dB) of a precise value.
Further, in the case where the Hanning window is used as the window function, if the signal frequency f coincides with the frequency f.sub.0 corresponding to the frequency point k.sub.0, three spectral lines are observed as shown in FIG. 6(a). However, if f=f.sub.0 +.DELTA.f/2, then the signal frequency is dispersed into a larger number of the spectral lines as shown in FIG. 6(b), so that the amplitude of the spectrum at the frequency point k.sub.0 is reduced to 85% (-1.4 dB) of the precise value.
Moreover, if the number of rotations of the rotating body (input signal frequency) fluctuates during an analysis period, the frequency spectrum of the input signal is dispersed into a still larger number of the spectral lines.
Since the conventional frequency spectrum analyzer displays only the amplitude of the spectrum at a specified frequency point, there is a problem in which the displayed values are reduced with large errors due to spectral dispersion caused by the above-mentioned leakage and fluctuations.